Nitrogen is produced by rectifying a nitrogen and oxygen containing gas, typically air, within a distillation column that is used in connection with a cryogenic rectification process. In such processes, the feed stream containing the oxygen and nitrogen is compressed and purified within a purification unit. The compressed and purified stream is then cooled within a main heat exchanger and introduced near the bottom of a distillation column.
Rectification of the feed stream within the distillation column produces a nitrogen-rich column overhead and an oxygen-rich column bottoms. Part of the nitrogen-rich vapor overhead is condensed within a heat exchanger that is operatively associated with the distillation column through indirect heat exchange with a stream of the oxygen-rich column bottoms. The condensation produces a nitrogen-rich liquid stream. A portion of such stream is returned to the column as reflux and another part can be taken as a nitrogen-rich liquid product stream. Additionally, another part of the nitrogen-rich vapor can either be further purified in an additional distillation column or can be directly taken as a product and fully warmed in the main heat exchanger in order to help cool the feed stream to a temperature suitable for its rectification.
The cryogenic rectification process has to be refrigerated in order to offset ambient heat leak, to maintain heat exchanger operation and to produce liquid products. Typically in a nitrogen plant, a stream of oxygen-rich vapor produced by partial vaporization of the stream of the oxygen-rich liquid column bottoms within the heat exchanger used in condensing the nitrogen-rich vapor is partially warmed and then expanded in a turboexpander that can be used to generate shaft work which can be employed to generate electricity. The turboexpansion produces an exhaust stream that is substantially warmed in the main heat exchanger in order to impart refrigeration to the process. Alternatively, air expansion can be used in which the feed air stream is compressed in a base load compressor and optionally a separate booster air compressor. Thereafter, the feed air stream is partially cooled within the main heat exchanger and all or part of the air is introduced into a turboexpander. In such case, refrigeration is imparted by introduction of the exhaust stream from the turboexpander into the distillation column. Another part of the compressed feed air stream can be fully liquefied within the main heat exchanger and also introduced nearer the bottom of the distillation column.
In many instances, it is desirable to produce variable quantities of the liquefied nitrogen both for back up and for merchant export. Additionally, the increasing prevalence of “real time” power pricing is forcing the introduction of increased flexibility into cryogenic rectification plants. In addition, liquid production must often be accomplished while a gaseous product flow is at or near the design rate. To produce variable quantities of liquid product refrigeration production must be varied.
U.S. Pat. No. 3,492,828 discloses a process in which a portion of the product nitrogen stream is compressed in an expander coupled to a booster compressor and recycled to the cold box. A variable amount of nitrogen can be recycled to the cold box and the turbine to vary the refrigeration and hence the liquid production. The disadvantage of such a modification is that it complicates the heat exchanger network, for example, by the requirement that passages be added, and the increased expansion flow rate results in a greater difficulty in maintaining high turboexpansion efficiency. In this regard, radial inflow turbine efficiency can be related to the ratio of volumetric flow rate and the square root of adiabatic head. In general, increasing the flow rate alone will negatively impact turbine efficiency.
A method of varying liquid nitrogen production is disclosed in U.S. Pat. No. 4,566,887. In this patent, a bypass line is used to periodically redirect a portion of the feed air into either a turboexpander that is used to expand the oxygen-rich vapor generated through condensing the nitrogen-rich vapor column overhead. The stream of the oxygen-rich vapor can also be introduced into a separate expander. In such manner, more refrigeration is produced to allow a greater production of the liquid nitrogen product. The problem with this approach is that since part of the feed air is redirected away from the column, during such a redirection, there would be a precipitous decline in gaseous nitrogen production.
As will be discussed, the present invention relates to a method of producing a liquid product by a method in which the expansion ratio across a variable speed turboexpander is varied to adjust the refrigeration and therefore the production of the liquid product while maintaining the turboexpander efficiency essentially constant by varying the flow rate to the variable speed turboexpander. Among other advantages, the present invention allows more variability in the production of product slates than the prior art, for example, the production of both liquid and vapor products.